目录结构

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参考文献

I Introduction▲
2 Experimental▲
3 Results and discussion▲
3.1 Microstructure
3.2 Thermoelectric properties
4 Conclusion▲
图表▲

Rare Metals2019年第12期

Thermoelectric properties of Bi_0.5Sb_1.4-xNa_xIn_0.1Te₃ alloys

Yue-Zhen Jiang Xing-Kai Duan

School of Electronic Engineering,Jiujiang University

School of Mechanical and Materials Engineering,Jiujiang University

作者简介：*Xing-Kai Duan e-mail:duanxingkai@163.com;

收稿日期：12 July 2019

基金：financially supported by the National Natural Science Foundation of China (No.51161009);

Thermoelectric properties of Bi_0.5Sb_1.4-xNa_xIn_0.1Te₃ alloys

Yue-Zhen Jiang Xing-Kai Duan

School of Electronic Engineering,Jiujiang University

School of Mechanical and Materials Engineering,Jiujiang University

Abstract：

The Bi_0.5Sb_1.4-xNa_xIn_0.1Te₃(x=0.02-0.20)alloys were fabricated by high vacuum melting and hotpressing technique.The phase structures and morphology of the bulk samples were characterized by X-ray diffraction(XRD) and scanning electron microscope(SEM),respectively.Effects of In and Na co-doping on the electrical and thermal transport properties were studied from room temperature to 500 K.Seebeck coefficient of the Bi_0.5Sb_1.5Te₃ can be enhanced by substituting Sb with In and Na at near room temperature.The electrical conductivity of the In and Na co-doped samples is lower than that of the Bi_0.5Sb_1.5Te₃ alloy from room temperature to 500 K.In and Na co-doping of appropriate percentage optimizes the thermal conductivity of the Bi_0.5Sb_1.5Te₃ alloy.The minimum value of thermal conductivity of Bi_0.5Sb_1.36Na_0.04In_0.1Te₃ alloy is 0.45 W·m^-1 K^-1 at 323 K.which leads to a great improvement in the thermoelectric figure of merit(zT).The maximum zT value reaches 1.42 at 323 K.

Keyword：

Microstructure; Co-doping; Thermal conductivity; Electrical properties;

Received： 12 July 2019

I Introduction

Thermoelectric materials are used to convert waste heat into electricity or electrical power directly into heating and cooling [ 1, 2, 3, 4, 5] .Bi₂Te_3-based alloys are the most important commercial thermoelectric materials near room temperature.Recently,high thermoelectric performance of Bi₂Te₃-based materials has been achieved by using many methods and strategies such as hot deformation [ 6, 7, 8, 9] ,point defects [ 10] ,nanocomposites [ 11, 12, 13] ,nanostructures [ 14, 15, 16, 17, 18, 19, 20] and doping [ 22] .Highefficiency thermoelectric materials require a large Seebeck coefficient,high electrical conductivity and low thermal conductivity.Owing to interdependence of the thermoelectric parameters,increasing the thermoelectric figure of merit (zT)value remains a challenge.It is well known that doping may determine thermoelectric properties of the thermoelectric materials.The alkali atoms tend to have soft rattling-type phonon modes,which results in very low thermal conductivity in Bi₂Te₃-based materials [ 23] .Ji et al. [ 24] had developed a novel alkali metal salt hydrothermal nanocoating treatment method to coat the polycrystalline P-type Bi₂Te₃ bulk grains with a thin layer,and all three constituent physical properties of zT are“decoupled”and can be modified inpidually by different alkali metal salt solutions.On the other hand,alkali metals doping can effectively improve the Seebeck coefficient.Substituting Ag with Nahas great influence on electron band structure and can enhance the Seebeck coefficient effectively,while adding excess Te in Na-doped samples plays a great role in optimizing the electrical and thermal conductivity in AgSbTe₂ compound [ 25] .Substituting three valence electrons for Sb with five valence electrons in the Bi_0.5Sb_1.5Te₃ alloy can be expected to be beneficial to the optimization of the carrier concentration,which results in an improvement in Seebeck coefficient.

To research the feasibility of the co-doping to enhance thermoelectric properties of the Bi₂Te₃-based materials,In and Na double partial substitutions for Sb in the Bi_0.5Sb_1.5Te₃ alloys have been done.In present work,the samples were prepared by vacuum melting and hot-pressing technique.The effects of In and Na co-doping on electrical and thermal transport properties were investigated.

2 Experimental

Elemental powders with In (99.99%),Bi (99.99%),Sb(99.99%),Te (99.99%) and bulk Na (99.5%) were weighed according to the atomic ratios of Bi_0.5Sb_1.5Te₃,Bi_0.5Sb_1.4-xNa_xln_0.1Te₃ (x=0.02,0.04,0.06,0.08,0.10,0.20).They were charged into a vacuum quartz tube,respectively.The elemental mixtures were melted at 1073 K for 8 h.Then,they were cooled to room temperature in the furnace.The gained ingots were pulverized using the agate mortar in atmosphere environment.The sizes of powders were controlled using the 300-mesh sieve.The powders were hot-pressed in the graphite dies under 50 MPa at 753 K for1 h.Bulk disk-shaped pellets with dimension of 12 mm x2 mm were prepared by hot pressing.

The phase structure and microstructure were analyzed by X-ray diffractometer (XRD,Bruker,D8 Advance with Cu Kαradiation,λ=0.15406 nm) and scanning electron microscope (SEM,VEGA-II LSU),respectively.The Seebeck coefficients (S) were achieved by testing thermoelectric voltage under a temperature gradient with an accuracy of±7%from 300 to 500 K.The electrical resistivity (p) was measured by four-point probe from 300to 500 K.The measure error of electrical resistivity is about±5%.The electrical properties were tested perpendicular to the hot-pressing direction.The thermal diffusivity (λ) was tested by the laser flash diffusivity method using a laser flash method (LFA 457).The thermal diffusivity was tested at the parallel to the hot-pressing direction.The measure error of thermal conductivity is about±5%.The heat capacity (C_p) was obtained from differential scanning calorimetry (DSC,DSC-Q20).The thermal conductivity was calculated according to the equation:λ=κ/(DC_p),whereλis the thermal diffusivity,D is the density of samples,C_p is the heat capacity,andκis the thermal conductivity.Hall coefficients were measured by the bar method using a Hall measurement system (Accent HL5500PC) at 300 K.The applied magnetic field is perpendicular to the hot-pressing direction.

3 Results and discussion

3.1 Microstructure

XRD patterns of Bi_0.5Sb_1.5Te₃ and Bi_0.5Sb_1.4-xNa_xln_0.1Te₃bulk samples are presented in Fig.1.All the characteristic peaks of the bulk samples can be indexed into rhombohedral Bi_0.5Sb_1.5Te₃ (JCPDS No.49-1713) phase with no impurities,which indicates the single-phase structure of the samples.The (001) peaks including (006),(009),(0015),(0018) and (0021) can be observed in the diffraction spectrums from section perpendicular to hot-pressing direction,which indicates that the basal planes are preferentially orientated perpendicular to hot-pressing direction.Since the rhombohedral Bi₂Te₃-based alloys have a quasilayered crystal structure,it is important to determine whether all the hot-pressed samples are textured or not.The orientation degree of the (00l) planes can be determined by the orientation factor f,which can be calculated using the following method:

where P and P₀ are the ratios of the integrated intensities of all (00l) planes to those of all (hkl) planes for the preferentially oriented and the randomly oriented samples,respectively.

where I₀(hkl) and I(hkl) are the peak integral intensities for randomly oriented sample and measured samples,respectively.The result of the orientation factor (f) was0.29±0.01.Because all the samples were prepared under the same condition,the values of f were almost equal.Therefore,it is concluded that the preferentially oriented caxis samples can be fabricated by the vacuum melting and hot-pressing method.

SEM images of the Bi_0.5Sb_1.5Te₃ and the Bi_0.5Sb_1.36-Na_0.04ln_0.1Te₃ samples are shown in Fig.2.Figure 2a,c shows a laminated structure composed of micro-nanolayers.The Bi_0.5Sb_u6Na_0.04ln_0.1Te₃ sample is found to have some randomly spaced pores in Fig.2d.The size of these pores ranges from 300 to 600 nm.The other samples also exhibit the similar micros true tures due to the same sintering method,but there are few randomly spaced pores in Fig.2b.The above results are also confirmed by the density of the samples,which are shown in Table 1.The density of Bi_0.5Sb_1.36Na_0.04ln_0.1Te₃ sample is the lowest among all the samples.

Fig.1 XRD patterns of Bi_0.5Sb_1.5Te₃ bulk sample and Bi_0.5Sb_1.4-x-Na-_xn_0.1Te₃ bulk samples

3.2 Thermoelectric properties

The temperature dependences of the Seebeck coefficient,the electrical conductivity and the power factor for the samples are shown in Fig.3.The effects of co-doping on the electrical transport properties of the Bi_0.5Sb_1.4-xNa_xIn_0.1Te₃ (x=0.02-0.20) were systematically investigated.All the samples exhibit the p-type semiconductor behavior within the whole test temperature range,as shown in Fig.3a.Figure 3a also shows that the In and Na co-doping leads to the increase in the Seebeck coefficient from 300 to350 K.This is attributed to the In and Na co-doping causing the change in the carrier concentration.As shown in Table 1,the co-doped samples result in the decrease in the hole concentration at room temperature.In this way,the Seebeck coefficients increase.Compared with the Bi_0.5Sb_1.5Te₃ sample,Fig.3b shows that the electrical conductivity of the In and Na co-doped samples is lower than that of the Bi_0.5Sb_1.5Te₃ alloy within the whole test temperature range.The carrier concentration and mobility are mainly two factors,which can lead to the differences of the electrical conductivity.The carrier concentration is mainly related to the composition,while the mobility can be affected by the grain size,grain boundary density and defects [ 26] .The incorporation of In and Na in the structure of Bi_0.5 Sb_1.5Te₃ results in the decrease in the hole concentration and mobility in Table 1.Owing to the lower electrical conductivity,the power factors of the co-doped samples are lower slightly than that of the Bi_0.5Sb_1.5Te₃sample,as shown in Fig.3c.

下载原图

Table 1 Density (D),hole concentration (n) and mobility (μ) of bulk Bi_0.5Sb_1.4-xNa_xIn_0.1Te₃ samples at room temperature

The thermal properties and zT values are shown in Fig.4.As shown in Fig.4a,the total thermal conductivity is the sum of contributions of the electronic thermal conductivity (κ_e) and the lattice thermal conductivity (κ_L).The electronic thermal conductivity (κ_e) is quantified through the Wiedemann-Franz law:κ_e=L₀T/p,where L₀ is the Lorenz number,T is the absolute temperature,and p is the electrical resistivity [ 27] .The classical value of the Lorentz number L₀=2.45×10^-8 V²·K^-2 and is valid for a degenerate electron gas.In this paper,L₀=2.0×10^-8 V²·K^-2 is taken,which is used for a heavily doped semiconductor [ 28] .The total thermal conductivity of the Bi_0.5Sb_1.4-xNa_xln_0.1Te₃ (x=0.02-0.06)samples is lower than that of the Bi_0.5Sb_1.5Te₃ sample from300 to 500 K.The alkali atoms tend to have soft rattlingtype phonon modes,which results in very low thermal conductivity.With the Na substitution fraction increasing from 0.08 to 0.20,the thermal conductivity of the Bi_0.5Sb_1.4-_xNa_xIn_0.1Te₃ samples is larger than that of the Bi_0.5Sb_1.5Te₃ sample.Figure 4b shows that the electrical thermal conductivity of the Bi_0.5Sb_1.4-xNa_xln_0.1Te₃ samples is lower than that of the Bi_0.5Sb_1.5Te₃ sample from 300to 500 K.The observed decreases are associated with the decrease in the hole concentration and mobility,which are shown in Table 1.As shown in Fig.4c,the lattice thermal conductivity of the co-doping samples increases with Na substitution fraction increasing from 0.08 to 0.20.Because the electronegativity difference between Na and Te (1.17)is much larger than that between Sb and Te (0.05),replacing Sb with Na should result in much stronger bonding between anions and cations.So,the anhanmonicity of the lattice vibrational spectrum is decreased and a rise in k_L can be expected.With Na substitution fraction increasing from 0.02 to 0.04,the Bi₀.5Sb_1.36Nax ln_0.1Te₃samples show lower over the whole temperature region.The x=0.04 sample has the lowest lattice thermal conductivity.Besides solid solutions scattering high frequency phonons,decrease of the lattice thermal conductivity may be caused by the randomly spaced pores.The spaced pores are shown in Fig Fig.2d.The randomly spaced pores are beneficial to the scattering of phonons,which contributes to the observed lower lattice thermal conductivity [ 29, 30, 31] .Figure 4d shows temperature dependence of zT value of all the samples.For the Bi_0.5Sb_1.5Te₃ sample,the zT value of1.01 is obtained at 323 K.Compared with the Bi_0.5Sb_1.5Te₃sample,the zT values of the Bi_0.5Sb_1.4-xNa_xln_0.1Te₃ samples decrease with the increasing Na substitution fraction from x=0.06-0.2 in the whole temperature range.The Bi_0.5Sb_1.36Na_0.04In_0.1Te₃ sample shows the highest zT values from 300 to 450 K.The increases in zT values can be attributed to the lower thermal conductivity.The zT value of 1.42 is obtained at 323 K.The value represents40%enhancement with respect to the Bi_0.5Sb_1.5Te₃ sample at the same temperature.

Fig.2 SEM images of bulk samples:a Bi_0.5Sb_1.5Te₃ from sections parallel to hot-pressing direction;b Bi_0.5Sb_1.5Te₃ from sections perpendicular to hot-pressing direction;c Bi_0.5Sb_1.36Na_0.04In_0.1Te₃ from sections parallel to hot-pressing direction;d Bi_0.5Sb_1.36Na_0.04In_0.1Te₃ from sections perpendicular to hot-pressing direction

Fig.3 Temperature dependence of electrical properties of Bi_0.5Sb_1.4-xNa_xIn_0.1Te₃ (x=0.02-0.20) samples:a Seebeck coefficient (S);b electrical conductivity (σ);c power factor (PF)

Fig.4 Temperature dependence of thermal conductivity of Bi_0.5Sb_1.4-xNa_xIn_0.1Te₃ (x=0.02-0.20) samples:a total thermal conductivity (κ_T);b electrical thermal conductivity (κ_E);c lattice thermal conductivity (c_L);d figure of merit (zT)

4 Conclusion

XRD results show that all the characteristic peaks of the bulk samples can be indexed into rhombohedral Bi_0.5Sb_1.5Te₃ phase with no imapurities.It is concluded that the preferentially oriented c-axis samples can be fabricated by the vacuum melting and hot-pressing method.SEM results show a laminated structure composed of micronanolayers.Compared with the Bi_0.5Sb_1.5Te₃ sample,the In and Na co-doped Bi_0.5Sb_1.5Te₃ samples bring about substantial increase in Seebeck coefficient at near room temperature.The electrical conductivity of the In and Na co-doping samples is lower than that of Bi_0.5Sb_1.5Te₃ alloy within the whole test temperature range.For the Bi_0.5Sb_1.36Na_0.04In_0.1Te₃ sample,a minimal thermal conductivity of 0.45 W·m^-1·K^-1 is obtained,with the maximal zT value reaching 1.42 at 323 K.The value represents40%enhancement with respect to the Bi_0.5Sb_1.5Te₃ alloy at the same temperature.The results show that the In and Na dual substitutions for Sb are effective in enhancing thermoelectric performance of Bi_0.5Sb_1.5Te₃ alloy.